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Relative Sea-Level (RSL) Fluctuations

Summary

Although relatively little sea-level research has been conducted on the central coast, a previous study (Andrews and Retherford, 1978) suggested that: i) RSL reached +120 m above modern levels on Calvert Island, ii) that during the Holocene sea levels fluctuated within +/- 20 m of the present shoreline, and iii) RSL was as much as -10 m below present for much of the last 7,000 years. Our research, in collaboration with Hakai archaeologists Duncan McLaren and Daryl Fedje, aims to determine the magnitude and patterns of RSL change since deglaciation, and close the knowledge gap.

Map of the Pacific coast of North America showing hypothesized ‘sea level hinge’ zone between areas that were isostatically depressed and areas that were uplifted as a result of a glacial forebulge during the Last Glacial Maximum (from Shugar et al 2014).
Progress Updates

To improve our understanding of sea-level dynamics and the coastal evolution of Calvert Island, we compiled and standardized a database of over 2,200 radiocarbon-dated sea level indicators to provide a regional synthesis of RSL changes since the LGM spanning the coast from northern California to Alaska (see Fig. 1). Our findings show that RSL changes in western North America are complex and heterogeneous owing to regional differences in crustal deformation (neotectonics), changes in global ocean volumes (eustasy) and the depression and rebound of the Earth's crust in response to ice loading (isostasy). The study (Shugar et al. 2014) was published in Quaternary Science Reviews and can be found here.

At the LGM, the Cordilleran Ice Sheet depressed the crust and created a raised forebulge along peripheral areas offshore. This, combined with different tectonic settings along the coast, resulted in divergent RSL responses since glaciation (Fig. 1). For example, sea level was up to 175 m higher than present in the lower Fraser Valley of southwestern BC, due largely to isostatic depression. At the same time, RSL was 150 m lower than present in Haida Gwaii about 250 km north of Calvert Island due to the combined effects of a glacial forebulge raising the land with lower eustatic sea level. A forebulge also developed in parts of southeast Alaska resulting in post-glacial RSL at least 122 m lower than present and possibly as low as 165 m.

An interesting finding of this study is the existence of a “hinge line” (Fig. 4) between isostatically depressed inner coast and forebulged outer coastal locations, where post-glacial changes in RSL were minimal. The interpreted geographical extent of this zone is from south-central Alaska through northern Vancouver Island and passes directly through Calvert Island. Subsequent research (e.g., Mann et al., 2014, Mann 2014) confirmed the location of this hinge in southeast Alaska, and suggested that it may have provided favourable conditions for travel of early humans along the coast of North America. Our work with Duncan McLaren and the archaeology program of the Hakai Institute has shown that the sea-level history of the Calvert Island region follows this hypothesized pattern of stable post-glacial RSL, at least over the past 15,000 years (Fig. 5). These remarkable findings indicate a relatively stable coastal environment amenable to long-term human occupation and settlement in this region. This study (McLaren et al. 2014) was published in Quaternary Science Reviews and can be found here.

Relative sea level curve for the Hakai West region of the study area. Data points are coded as green (above higher high tide), red (intertidal), blue (subtidal) (from McLaren et al. 2014).